Then the sequence \(\lbrace \phi _k\rbrace _{k=1}^\infty \)  forms the standard orthonormal basis in \(L^2(\Omega )\) .
We define the Mittag-Leffler function [1]} by
\(E_{\alpha ,\beta } = \sum _{k=0}^\infty \frac{z^k}{\Gamma (\alpha k+\beta )},\quad z\in \mathbb {C},\nonumber \) 
